Field of the Invention
The present disclosure generally relates to signal processing, and in particular it relates to signal processing of at least one embodiment of a focus detection apparatus, control method therefor and storage medium for use therewith.
Description of Related Art
Focus detection, and in particular autofocusing (AF) can be achieved by phase detection or contrast measurement. Phase detection is achieved by dividing incoming light into pairs of image signals and comparing them by correlation calculation. Conventionally, when correlation calculation for focus detection using a phase difference system is performed, it is known that there is a case where a pair of image signals become unsymmetrical to each other (also referred to as image signal distortion) when the correlation calculation is performed in a state where an image is blurred. When the correlation calculation using the pair of image signals in which the image signal distortion is generated is performed, it is also known that, depending on conditions of an object and a blur, there is a case where a false in-focus state in which a correlation calculation result indicating as if an in-focus state was achieved despite the blur is obtained is provided.
Japanese Patent Application Laid-Open No. 2014-219549 discloses a technique for focus detection using a phase difference system, as follows. First, filter processing for performing edge extraction for cutting a DC component of each of a pair of image signals is performed. Then correlation calculation is performed, and on the basis of a correlation calculation result using the pair of image signals subjected to the filter processing, whether or not a focus state is an in-focus state is determined. When the focus state is determined to be the in-focus state, filter processing for suppressing a high frequency noise component without cutting a DC component is further performed for each of the pair of image signals. Then, on the basis of a result of the correlation calculation using the pair of image signals subjected to the filter processing, whether or not a false in-focus state exists is determined.
In the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2014-219549, even when the filter processing for suppressing the high frequency noise component is performed, for example, in a case where the DC components of the pair of image signals have approximately the same value, there is still a possibility that the focus state is determined to be an in-focus state despite being a false in-focus state.